1. Field of the Invention
The invention relates to a power supply module for an implantable device, the power supply module encompassing a biocompatible outer housing which holds a repeatedly rechargeable electrochemical battery that supplies electrical power to the main module of the implantable device via a coupling element.
2. Description of Related Art
U.S. Pat. No. 5,279,292 discloses an implantable device which is a hearing aid or a tinnitus masker which has, in one embodiment, a main module and a power supply module. The two modules are each accommodated in a separate biocompatible housing, power transmission from the power supply module to the main module taking place via a coupling element with a metallic or metallically separated and inductively coupled connection. The housing of the power supply module can hold a battery, charging electronics and a receiving resonant circuit which can be inductively coupled to a transmitting resonant circuit of a charging means which can be attached outside the body. One important advantage of the modular structure is that, with the implantation site of the power supply module, the individual is not linked to that of the main module. Rather the power supply module can be implanted anywhere on the body where there is enough space, in addition a battery with relatively large electrical capacitance can be used. This applies to a coupling element which is made for a permanent connection in the same way as for a detachable coupling element. The latter, at the same time, allows replacement of the battery without the need to replace the entire system.
The coupling element which is disclosed in U.S. Pat. No. 5,279,292 and which is made for a metallically separated and inductively coupled detachable connection comprises two coupling coils and a ferrite rod as the common core. One coupling coil is assigned to the power supply module and is supplied as part of a serial tuned circuit from the battery via an oscillator, the second coupling coil, which acts as the receiving coil, is connected to the main module via a flexible connecting lead. The AC voltage induced in the receiving coil is available via a rectifier to operate the hearing aid.
German patent disclosure document DE3 31 620 A1 describes a hermetically tight, plug-and-socket connection which is used for a detachable metallic connection of the electrode feed lines to an implantable pacemaker, a defibrillator or a cardioverter.
U.S. Pat. No. 5,755,743 relates to a contact arrangement for a detachable electrical connection between an implant housing and other, especially sensor and actuator components, with which a high degree of miniaturization can be achieved.
One special problem in the use of repeatedly rechargeable electrochemical batteries is that, in case of excess charging or a short circuit between the terminal contacts or poles of the battery, a pressure rise within the battery housing can occur which leads to its deformation which, in turn, can become so great that chemicals, especially in gaseous form, emerge.
Published European Patent Application 0 322 112 (corresponding to U.S. Pat. No. 4,756,983), published European Patent Application 0 360 395 (corresponding to U.S. Pat. No. 4,937,153) and published European Patent Application 0 370 634 (corresponding to U.S. Pat. No. 4,871,553) disclose providing electrochemical batteries with a switching element which, when a certain tolerated boundary deformation of the battery housing is exceeded, preferably, irreversibly breaks an electrical terminal contact away from the pertinent electrochemically active electrode in order to prevent further deformation of the battery housing. The battery housing comprises an electrically conductive cylindrical housing segment closed on the face, and in contact with an electrode, and on the face, the plate-shaped switching element being attached centrally by means of an electrically insulating cement from the outside. The electrically conductive switching element, in its base position, forms above its outside edge an electrical connection between the housing segment and the electrical terminal contact which projects to the outside and which is located in the center of the switching element. When the pressure rises within the battery housing, the face of the housing segment which acts as a detector element arches to the outside, causing contact to be interrupted between the housing segment and the outside edge of the switching element, and thus, between the electrode and the terminal contact. Typical applications of these switching elements are type "D" standard batteries. The curvature of the face, starting from which the switching element breaks the electrical contact, in this case, is 0.76 mm to 1.8 mm. For a curvature of more than 1.8 mm, leakage of chemicals from within the battery can usually be expected.
Published European Patent Application 0 470 726 discloses an electrochemical battery with a cylindrical battery housing and a pressure membrane as the detector element which is integrated on the face in the battery housing and which curves when the pressure rises within the battery housing, by which a plate-shaped switching element, which is connected to the pressure membrane in the center, reversibly or irreversibly interrupts the electrical contact between an electrode and a terminal contact of the battery.
Published European Patent Application 0 674 351 (corresponding to U.S. Pat. No. 5,585,207) discloses an electrochemical battery with a battery housing which comprises a cutting device which can be actuated by a pressure membrane and which irreversibly breaks an electrical conductor which connects the terminal contact of the battery with an electrochemically active electrode when a boundary pressure within the battery housing is exceeded.
When a switching element is being used which breaks the electrical connection between a terminal contact and the associated electrochemically active electrode when a certain pressure within the battery housing is exceeded, it is possible for the pressure to continue to increase and ultimately for chemicals to discharge from the battery housing or even for it to explode. For this reason, it was proposed (for example, in Published European Patent Applications Nos. 0 364 995, 0 573 998 or 0 739 047, which correspond to U.S. Pat. Nos. 4,943,497, 5,418,082 and 5,766,790, respectively) that a pressure membrane which actuates the switching element and which is located in the battery housing be provided with a bursting area via which after activation of the switching element and a further pressure increase chemicals can emerge from within the battery housing.
The safety measures cited in the aforementioned prior art for electrochemical batteries are not adequate or are unsuited for use in a power supply module of implantable devices, since for this purpose, especially high demands, particularly with respect to safety and reliability, must be satisfied with, at the same time, a reduction of all dimensions to the largest degree possible.